The present invention relates to the art of radio receiver systems, and more particularly to systems for receiving single channel, amplitude modulated signals representing stereophonic program signals.
The broadcasting community has long sought an acceptable system for transmitting and receiving stereophonic signals in the AM frequency band. Although many systems have been proposed, non has yet met with general acceptance. Several of these schemes have proposed modulating the two program signals onto differently phased carriers which are linearily combined into a single composite stereo signal before being transmitted. Some of the proposed systems have set the phase angle between the two carrier signals to be approximately 90.degree., thus creating a standard quadrature modulation scheme. Other systems have also been proposed wherein the phase angle between the two carrier signals is modified to be less than 90.degree.. Modified quadrature modulation schemes of this nature are disclosed in the following patents: Collins, Jr., et al., U.S. Pat. No. 3,231,672; Barton, U.S. Pat. No. 3,102,167; and in my co-pending patent application, Ser. No. 812,657, filed on July 5, 1977.
To receive a signal modulated in this fashion, synchronous detection schemes must be employed, such as described in the patent to Collins, Jr., U.S. Pat. No. 3,043,914 and the patent to Colodny, U.S. Pat. No. 3,013,529. In order to provide optimum reception, it is necessary that a carrier signal be provided which is synchronized with the incoming carrier in both phase and frequency. This synchronism must be quite exact in order to obtain maximum separation between the recovered stereo signals. Although this constraint can be relaxed to some extent if a standard quadrature modulation scheme is employed, this approach is not desirable since it has been found that compatibility with existing monophonic AM receivers requires that this phase angle be considerably smaller. As the phase angle is reduced from 90.degree. to much smaller angles, however, the exactness and stability of the synchronous detection becomes even more critical.
Moreover, conventional AM detection schemes have employed intermediate-frequency (IF) stages having very narrow bandpass characteristics in order to improve the selectivity of the AM receiver. When these techniques are utilized in detecting the modulated signals referred to previously, however, the bandpass of the IF stage must be symmetrical on either side of the composite stereo signal which is being demodulated or else further degradations in stereo separation will result.